You get the most out of a FF program when flying the carrier that uses that program. UA does not fly TATL from JFK.
Also, how long is this change, is it "permanent" or just a few years?
And what is the goal of being a part of a FF program? Which potential benefits?
What cabin will the regular flights?
There is a FT forum / thread specifically aimed at this query,
Which Frequent Flyer Program to Join? Help Is Here!
So will move your query to that forum.
WineCountryUA
UA coModerator